cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/experiment/hirl/hirl.go

@@ -251,7 +251,7 @@ func (meth squidCacheManager) Run(ctx context.Context, config MethodConfig) {
 type RunMethodConfig struct {
 	MethodConfig
 	Name        string
-	NewDialer   func(config netx.Config) netx.Dialer
+	NewDialer   func(config netx.Config) model.Dialer
 	RequestLine string
 }